Magnetic field probe for determining a disposition of an implantable magnetic marker

ABSTRACT

During both invasive and non-invasive treatments and therapies, inaccuracies in locating the areas of interest mean that not all the area is treated, or the treatment is incomplete. A magnetic field probe  100, 1010, 102, 103  is provided that improves determination of a disposition of an implantable magnetic marker  200 , the probe comprising a first  110, 120  and second  110, 120  magnetic sensor, substantially disposed along a transverse axis intersecting the longitudinal axis of the probe  150 . The first  110, 120  and second  110, 120  magnetic sensors are close to the distal end  160  of the probe, and are separated by a minor sensor separation. A third  120, 130  magnetic sensor is provided close to the proximal end  165 , separated by a major sensor separation from the second magnetic sensor  110, 120  close to the distal end  160 , the major sensor separation being larger than the minor sensor separation; and the ratio of the major sensor separation to the minor sensor separation is in the range 1.25 to 40, preferably in the range 1.6 to 7.6. 
     In this example, the second magnetic sensor is functionally configured and arranged to co-operate with both the first magnetic sensor and the third magnetic sensor. This may be implemented using three or more magnetic sensors. 
     This provides a probe capable of accurately determining one or more dispositions of the implantable magnetic marker when the distal end of the probe is close to the marker and also when it is further away. 
     In particular, including the pair of sensors close to the distal end may increase the sensitivity and accuracy of the probe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Track One Continuation of PCT Patent ApplicationNo. PCT/NL2019/050708 having International filing date of Oct. 28, 2019,which claims the benefit of Netherlands Patent Application No. NL2022093, filed Nov. 29, 2018, the contents of which are all incorporatedherein by reference in their entirety.

FIELD

The present disclosure relates to a magnetic field probe for determininga disposition of an implantable magnetic marker, a detection unitcomprising the probe and a method of detecting the disposition of animplantable magnetic marker.

BACKGROUND

During both invasive and non-invasive treatments and therapies, it isimportant that health professional be able to accurately locate areas ofinterest. Frequently, professionals rely on sight and manualmanipulation to find and remember areas of interest, often marking anouter surface of skin. In practice, imaging equipment such as X-rayand/or ultrasound, may also be used to assist in the location—however,this relies on being able to distinguish the area of interest from thesurrounding tissue using the imaging technologies. Inaccuracies in beingable to locate the areas of interest may mean that not all the area istreated, or the treatment is incomplete. This is a problem for boththerapeutic and cosmetic procedures and treatments, including removal oftumors, removal of polyps, cosmetic surgery, removal and/or correctionof tissues, localization of implanted devices—for example, birth-controldevices such as Implanon, may need to be localized.

For example, if lesion resection or removal is prescribed followingcancer screening, the surgeon needs to know the location and extent ofthe lesion. The current golden standard in clinical practice requiresthe placement of metal anchor wires in the target immediately before thesurgical procedure, which risks infection and movement of the wires.Newer solutions use radio-active markers, but the use of radio-activematerials is tightly controlled and regulated. Electro-magnetic and RFID(Radio-Frequency Identification) markers have been developed, but theseare bulky and prone to failure. Any inaccuracy in locating the area ofinterest may result in an incomplete resection or removal of the lesion,requiring additional treatments.

In addition, improvements in screening procedures means that smaller andearly stage lesions are increasingly being identified inpatients—although this early detection is more beneficial to thepatient, small lesions may be difficult for the surgeon to identify andlocate. They are also likely to be impalpable. Intra-operative imagingis often cumbersome and expensive.

Recently, the use of implantable magnetic markers has been proposed.These provide a higher degree of safety compared to radio-activemarkers, but it still requires considerable effort by the healthcareprofessional to detect the disposition (localize) of the marker. Thisbecomes even more difficult when very small magnetic markers are used tomark very small areas of interest.

For example, U.S. Pat. No. 7,561,051B1 describes an apparatus forlocating a magnet and/or determining the orientation of the apparatusrelative to the magnet. In one embodiment, the apparatus includes amulti-axis magnetic field sensor movable in a reciprocating manner so asto permit sensor readings at multiple spaced locations. In anotherembodiment, the apparatus includes a plurality of multi-axis magneticfield sensors arrayed along a straight line. The apparatus may be usedin a number of medical and other applications, including tissueresection, tracking movement of a medical device in a body cavity andtracking movement of an internal organ.

It is an object of the invention to provide improved determination of adisposition of an implantable magnetic marker.

GENERAL STATEMENTS

According to a first aspect of the present disclosure, there is provideda magnetic field probe for determining a disposition of an implantablemagnetic marker, the probe extending along a probe longitudinal axis,the probe comprising a distal end, configured and arranged to bedisposed close to an outer surface of skin; a first magnetic sensorclose to the distal end; a second magnetic sensor, close to the distalend, configured and arranged to be separated by a minor sensorseparation from the first magnetic sensor, the first and second magneticsensors being configured and arranged to determine, in use, one or moredispositions of the magnetic marker; a third magnetic sensor close to aproximal end, configured and arranged to be separated by a major sensorseparation from the second magnetic sensor, the third and secondmagnetic sensors being configured and arranged to further determine, inuse, the one or more dispositions of the magnetic marker; wherein: themajor sensor separation is larger than the minor sensor separation; andthe ratio of the major sensor separation to the minor sensor separationis in the range 1.25 to 40, preferably in the range 1.6 to 7.6; and thefirst and second magnetic sensors are substantially disposed along atransverse axis, the transverse axis intersecting the probe longitudinalaxis.

Note that the use of the labels first, second, and third for the sensorsis distinct and not necessarily the same as the labels first, second andthird for the sensor groups. The first, second and third sensors may beselected from any of those groups in accordance with the functionalityperformed by the different embodiments.

By providing, close to the distal end of the probe, a pair of magneticsensors with a minor separation along a transverse axis, and a furtherpair with a major separation, one close to the distal end and the otherclose to the proximal end, where the ratio of the major sensorseparation to the minor sensor separation is in the range 1.25 to 40,preferably in the range 1.6 to 7.6, a probe is provided capable ofaccurately determining the location (disposition) of the implantablemagnetic marker when the distal end of the probe is close to the markerand further away. In particular, including the pair of sensors close tothe distal end may increase the sensitivity and accuracy of the probe.

In this example, the second magnetic sensor is functionally configuredand arranged to co-operate with both the first magnetic sensor and thethird magnetic sensor. This may be implemented using three or moremagnetic sensors, as explained below.

Optionally, the transverse axis may be approximately perpendicular tothe longitudinal axis. This may simplify calculations of dispositions insome cases.

According to a further aspect, the probe may further comprise a fourthmagnetic sensor, close to the distal end, wherein the fourth magneticsensor is configured and arranged, instead of the second magneticsensor, to be separated from the third magnetic sensor by the majorsensor separation.

It may be convenient to provide an additional (fourth) magnetic sensor,which performs part of the functions of the second sensor in the firstaspect, namely co-operating with the third magnetic sensor. The secondmagnetic sensor is configured and arranged to be separated by a minorsensor separation from the first magnetic sensor.

According to another aspect of the current disclosure, the magneticsensors configured and arranged to be separated by the major sensordisposition, may be substantially disposed along a longitudinal axis.

Optionally, these sensors may be disposed along the probe longitudinalaxis.

Different pairs of magnetic sensors may be configured and arranged to beseparated by the major sensor disposition. By disposing such a pair ofsensors substantially along a longitudinal axis of the probe, a long,thin probe is provided which is particularly easy to manipulate. Bydisposing them along the probe longitudinal axis, calculations ofdispositions relative to the probe longitudinal axis may be simplifiedin some cases.

Optionally, the transverse axis may be approximately perpendicular to alongitudinal axis along which sensors are configured and arranged to beseparated by a major sensor separation. Optionally this may be the probelongitudinal axis.

This may simplify calculations of dispositions in some cases.

According to yet another aspect of the current disclosure, the probe maycomprise the probe further comprises: one or more compensation sensorsfor measuring a background magnetic field; wherein: the determination,in use, of one or more dispositions of the magnetic marker furtherconsiders the background magnetic field.

Advantageously, an existing sensor or a dedicated sensor may beconfigured to measure (or detect) a background magnetic field, such asthe Earth's magnetic field. The disposition determination may becompensated using background measurements to further increase theaccuracy and sensitivity.

According to another aspect of the current disclosure, the minor sensorseparation and/or the major sensor separation may be predetermined byconsidering the inverse cube law determination of a magnetic fieldstrength associated with the implantable magnetic marker.

By considering the inverse cube law when determining the longitudinalseparation of one or more sensors, the accuracy of the magnetic fieldmeasurement and the model and/or curve-fitting may be further improved.

In addition, a method is provided for determining the disposition of animplantable magnetic marker, the method comprising: providing a probecomprising a distal end, configured and arranged to be disposed close toan outer surface of skin, the probe extending along a probe longitudinalaxis and further comprising: a first magnetic sensor close to the distalend and a second magnetic sensor, configured and arranged to beseparated by a minor sensor separation from the first magnetic sensor,the first and second magnetic sensors being substantially disposed alonga transverse axis, the transverse axis intersecting the longitudinalaxis; configuring and arranging the first and second magnetic sensors todetermine, in use, one or more dispositions of the magnetic marker; theprobe further comprising a third magnetic sensor close to a proximalend, configured and arranged to be separated by a major sensorseparation from the third magnetic sensor; configuring and arranging thethird and second magnetic sensors to further determine, in use, the oneor more dispositions of the magnetic marker; wherein: the ratio of themajor sensor separation to the minor sensor separation is in the range1.25 to 40, preferably in the range 1.6 to 7.6.

In this example, the second magnetic sensor is functionally configuredand arranged to co-operate with both the first magnetic sensor and thethird magnetic sensor. This may be implemented using three or moremagnetic sensors, as explained below.

According to a still further aspect, the probe may further comprise afourth magnetic sensor, close to the distal end, the method comprising:configuring and arranging the fourth magnetic sensor, instead of thesecond magnetic sensor, to be separated from the third magnetic sensorby the major sensor separation.

It may be convenient to provide an additional (fourth) magnetic sensor,which performs part of the functions of the second sensor in the firstaspect, namely co-operating with the third magnetic sensor. The secondmagnetic sensor is configured and arranged to be separated by a minorsensor separation from the first magnetic sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of some embodiments of the present invention,and the manner in which the same are accomplished, will become morereadily apparent upon consideration of the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings, which illustrate preferred and exemplary embodiments and whichare not necessarily drawn to scale, wherein:

FIGS. 1A and 1B depict an embodiment of a magnetic field probe fordetecting the disposition (localizing) of an implantable magneticmarker;

FIGS. 2A and 2B depict a second embodiment of magnetic field probe;

FIGS. 3A, 3B and 3C depicts examples of longitudinally-extended probeswith 3-dimensional sensor arrangements;

FIG. 4 depicts an example of a simple differential measurement showingthe correlation between signal and marker lateral displacement;

FIG. 5 shows the expected variation in the magnetic field of a firstmarker and some possible sensor separations;

FIG. 6 shows the expected variation in the magnetic field of a secondmarker and some possible sensor separations;

FIGS. 7A and 7B depicts an alternative sensor 2-dimensional arrangement;and

FIGS. 8A and 8B shows a further alternative sensor 2-dimensionalarrangement.

DETAILED DESCRIPTION

In the following detailed description, numerous non-limiting specificdetails are given to assist in understanding this disclosure. It will beobvious to a person skilled in the art that the computer processing partof the method may be implemented on any type of standalone system orclient-server compatible system containing any type of client, network,server, and database elements.

FIG. 1A and FIG. 1B depict a magnetic field probe 100 for detecting adisposition (localizing) of an implantable magnetic marker 200. Asdepicted, the magnetic marker 200 is implanted below an outer surface ofskin 300 to mark an area of interest—this may be a few millimeters or afew centimeters below 250 the outer surface of the skin. This may alsobe called depth 250. The magnetic field probe may extend along a probelongitudinal axis 150—when extended, it provides a convenient referencepoint for determining a disposition of the magnetic marker 200. Thisdisposition of the marker 200 may be expressed in any convenientparameter—however, a user may be particularly interested in beingprovided with a transverse distance 260 between the probe longitudinalaxis 150 and the magnetic marker 200, and the longitudinal distance 255,250 between the distal end 160 and the magnetic marker 200.

The marker may be implanted in any convenient way, such as by injection.The injection may be, for example, into soft tissue or organs, ordelivery via a bronchoscope to lung bronchii, or coloscope to colon, orintegrated in a swallowable device such as an electronic pill. Themethod of implantation may depend on, for example, the depth 250required, the subsequent procedure to be performed, the size of the areaof interest, the location of the area of interest, the type of tissue inthe area, and the type of tissue surrounding the area. It may beimplanted immediately before detection, or some time earlier.

Typically, a suitable magnetic marker 200 is approximately cylindrical:

-   -   a diameter of 1.45 mm, a length of 2.19 mm and a remnant field        (Br) of 1.43 T (Neodymium N52), or    -   a diameter of 1.75 mm, a length of 5 mm and a remnant field (Br)        of 1.43 T (Neodymium N52).

The probe 100 comprises a distal end 160, configured and arranged to bedisposed close to an outer surface of skin 300. The probe 100 is furtherconfigured and arranged to determine one or more dispositions (distance)between a probe reference and the magnetic marker 200, as describedbelow. The probe reference may be one or more points of the probe 100,one or more axes of the probe 100, one or more planes of the probe 100,or a combination thereof.

The probe 100 comprises one or more magnetic sensors, configured tomeasure one or more properties of a magnetic field (for examplestrength, direction, Bx, By, Bz) generated by the magnetic marker 200.These properties are used to determine the one or more dispositionsusing a software algorithm.

For example, it may be advantageous for the user if the distal end 160of the probe 100 was a reference point—in other words, if the probe 100is configured to determine the one or more dispositions with respect tothe distal end 160 of the probe 100. If the probe extends substantiallylongitudinally, then a long thin probe 100 is provided which isparticularly easy to manipulate, and finding the magnetic marker 200 bymoving the distal end 160 is more intuitive.

As depicted, the distal end 160 may be disposed at a distance 255 fromthe outer surface of skin 300—a spacer may be used to maintain a fixeddistance 255, or the distance 255 may be zero if the probe 100 isfurther configured and arranged to contact the outer surface of skin300. The probe 100 may be further configured and arranged to be pushedagainst the outer surface of skin 300 to create an indent which mayfurther reduce the distance between the distal end 160 of the probe 100and the magnetic marker 200. In general, the smaller the distancebetween the probe 100 and the magnetic marker, the greater the amplitudeof any signal measured. For some treatments, the probe 100 may furtherconfigured and arranged to be inserted through the outer surface of skin300 and/or into a body cavity to further reduce the distance betweenprobe 100 and marker 200. This may be, for example, via a surgicalincision or via a natural orifice.

The probe 100 may be comprised in a detection unit or device (notshown). It will be clear to the skilled person that functionalities fordetermining the one or more dispositions may be implemented in thehardware and software of the magnetic probe 100, or they be implementedin the hardware and software of the rest of the detector. Thefunctionalities may also be divided in any convenient way between themagnetic probe 100 and the rest of the detector unit.

A detection unit or device for a probe 100 may comprise one or more ofthe following:

-   -   an optional electrical and/or mechanical connection, configured        to attach to a proximal end 165 of the probe 100. It may be        advantageous to make the attachment releasable. The connection        may also be wireless, configured and arranged to allow at least        data transmission between the probe 100 and the rest of the        detector;    -   a power supply to provide energy to the probe magnetic sensors;    -   a processor, configured to collect magnetic sensor measurement        values, and to determine the dispositions using an appropriate        software algorithm;    -   optionally, a display may also be provided to indicate to the        user the results of the determination. Preferably, a transverse        and/or longitudinal distance to the magnetic marker 200 is        displayed, numerically and/or graphically. Additionally or        alternatively, a graphical representation may be provided        indicating a transverse and/or longitudinal direction.        Additionally or alternatively, audio feedback may also be        provided—this is described in more detail below. The distances        (dispositions) may be displayed, for example, as relative values        and/or absolute values. Audio feedback may be provided, for        example, similar to the way distance to an object is indicated        with an automobile parking sensor with different tones.

The probe 100 comprises three or more magnetic sensors. These may beconfigured and arranged to be comprised in up to three different groupsof sensors 110, 120, 130. As described below, each sensor may beconfigured and arranged to perform a single function or to contribute toa plurality of functions. In this example, three groups 110, 120, 130are identifiable based on their position relative to the distal end 160of the probe 100. FIG. 1B depicts the reference number assigned to eachmagnetic sensor for ease of reference—in this embodiment, a minimum ofthree magnetic sensors are provided, and typically six sensors are usedin a 1D array. The sensors are grouped as follows:

-   -   110: a first group of at least two magnetic sensors, close to        the distal end 160 of the probe 100. As depicted in FIG. 1B,        these may be sensors 0, 1 and 2. Not all sensors positions need        to be occupied—one or more sensor may be physically omitted or        disabled in software. These sensors are configured and arranged        such that adjacent sensors are longitudinally and transversely        separated from each other by a minor sensor separation, such        that at least two dispositions of the marker 200 may be        determined when the distal end 160 is relatively close to the        magnetic marker 200. Sensors 0 and 1 are separated by a minor        sensor separation, and sensors 1 and 2 are also separated by a        minor sensor separation. Sensors separated by a minor sensor        separation are configured and arranged to provide measurements        for accurately determining one or more dispositions of the        marker 200 when the distal end 160 is relatively close to the        magnetic marker 200. In practice, a sensor may approach the        magnetic marker 200 to within approximately 2.5 mm at the        closest—typically, a housing will be used to enclose the        magnetic probe 100 limiting the closest distance that the marker        may be approached. For example, when IC magnetic sensors are        used such as LGA-12 packages with dimensions of 2×2×0.7 mm, they        may be disposed with a gap of 0.5 mm, which provide a minor        sensor separation (approximated to the distance between the        center of the packages or approximately 2.5 mm). The sensors        separated by the minor separation may be optimized for accurate        marker 200 disposition determination of between 0.5 and 3 times        the length of the marker.

Note that non-adjacent sensors in this embodiment are separatedlongitudinally by a major sensor separation: sensors 2 and 0.

-   -   120: a second group of at least one magnetic sensor. As depicted        in FIG. 1B, this may be sensor 3 and 12. These are closer to a        proximal end 165 of the probe 100 than the first group 110 (in        other words, further away from the distal end 160). They are        mainly configured and arranged to provide measurements for        determining one or more dispositions (distance) when the distal        end 160 of the probe 100 is further away from the magnetic        marker 200. The disposition determined using measurements from        one or more sensors in the second group is compared to a        disposition determined using measurements from one or more        sensor from the first group 110—the magnetic sensor from the        second group 120 is separated from the magnetic sensor in the        first group 110 by a major sensor separation. This major sensor        separation is greater than the minor sensor separation. For        example, the following are separated by a major sensor        separation in a longitudinal direction:    -   sensors 3 and 0, 1 or 2;    -   sensors 12 and 0, 1, or 2    -   sensors 12 and 3    -   130: optionally, a third group of at least one magnetic sensor.        As depicted in FIG. 1B, this may be sensor 13. These are closer        to the proximal end 165 of the probe 100 than the second group        120 (in other words, further away from the distal end 160). They        may be configured and arranged:    -   as a compensation sensor. In this case, compensation sensors are        mainly configured and arranged to detect a background magnetic        field, such as a naturally-occurring magnetic field (from the        Earth), a man-made field present due to equipment being operated        in the environment where the measurements and determinations are        performed, and/or a diamagnetic field created by the tissue in        or around the area of interest.    -   alternatively or additionally, magnetic sensors in the third        group 130 may be configured and arranged to provide additional        measurements for determining one or more dispositions (distance)        when the distal end 160 of the probe 100 is further away from        the magnetic marker 200. magnetic sensors from the third group        130 are separated from the first group 110 by a major sensor        separation. This major sensor separation is greater than the        minor sensor separation, and may be similar, different or equal        to any of the other major sensor separations. For example, the        following are separated by a major sensor separation in a        longitudinal direction: sensors 13 and 0, 1, or 2    -   magnetic sensors from the third group 130 are separated from the        second group 120 by a major sensor separation. This major sensor        separation is greater than the minor sensor separation, and may        be similar, different or equal to any of the other major sensor        separations. For example, the following are separated by a major        sensor separation in a longitudinal direction: sensors 13 and 12        or 3.

The terms minor sensor separation and major separation should beinterpreted as general categories of separation—a probe 100 may beprovided having one or more minor sensor separations and one or moremajor sensor separations.

A minor sensor separation is in the order of magnitude of thelongitudinal length of the magnetic marker 200 to be localized,preferably 0.5 to 3 times this marker length. So, for example, whenlocalizing a marker 2.19 mm long, the minor sensor separation ispreferably 1.095 mm to 6.57 mm. When localizing a marker 5 mm long, theminor sensor separation is preferably 2.5 mm to 15 mm.

A major sensor separation is in the order of several times the magnitudeof the longitudinal length of the magnetic marker 200 to be localized,preferably 5 to 20 times this marker length. So, for example, whenlocalizing a marker 2.19 mm long, the minor sensor separation ispreferably 10.95 mm to 43.8 mm. When localizing a marker 5 mm long, theminor sensor separation is preferably 25 mm to 100 mm.

If a plurality of minor sensor separations is provided, they may besimilar, different or equal to each other. If a plurality of majorsensor separations is provided, they may be similar, different or equalto each other. However, the major sensor separation is larger than theminor sensor separation and the ratio of the major sensor separation tothe minor sensor separation is in the range 1.25 to 40, preferably inthe range 1.6 to 7.6.

An advantageous aspect is providing at least two magnetic sensors,separated by a minor sensor separation along a transverse axis—this mayprovide a high degree of accuracy and/sensitivity.

Such an “L-configuration” is not known in the art. For example, U.S.Pat. No. 7,561,051 FIG. 2D depicts a 2D configuration, but theapproximately transversely-separated sensors are close to the proximalend, and away from the distal end. This is done to keep the probe ofU.S. Pat. No. 7,561,051 narrow near the front (distal end), as explainedin col 6, lines 11 to 16 of that publication.

The magnetic sensors 0, 1, 2, 3, 12, 13 may be single measurementdevice, such as a 1D fluxgate measuring a single direction of magneticfield flux. One or more fluxgates may have substantially the sameorientation or all may have different orientations—as these orientationsand sensitivities are predetermined during the design, the measurementsignals from each sensor 0, 1, 2, 3, 12, 13 may be combined with therelative position of the sensors as pre-determined by the design, andone or more selected sensor separations, to determine a disposition ofthe magnetic marker 200. Alternatively or additionally, theorientations, separations and/or sensitivities may be measured tocalibrate an assembled (or partially assembled) probe. Thepre-determined and/or calibrated values may be used in a software modeland/or a lookup table to determine, in use, one or more dispositions ofthe magnetic marker 200 relative to a reference point, axis and/or planeof the probe 100.

The separation of sensors 0, 1, 2, 3, 12, 13 may be determined byphysical measurement of the distances between the center of the sensorpackage—for many applications this may be sufficiently accurate,especially if a further calibration of the probe 200 is performed. Asdepicted in FIG. 1A, the center of the package for sensor 0 lies on afirst transverse axis 181, for sensor 1 on a second transverse axis 182,for sensor 2 lies on a third transverse axis 183, for sensor 3 lies on afourth transverse axis 184, for sensor 12 on a fifth transverse axis185, and for sensor 13 lies on a sixth transverse axis 186. Thetransverse axes 181 to 186 are substantially perpendicular to thelongitudinal probe axis 150.

The sensor separation is preferably determined in three degrees offreedom, although in some applications one degree of freedom or twodegrees of freedom may be sufficiently accurate.

As depicted in FIG. 1A, the magnetic sensors 0, 1, 2, 3, 12, 13 mayoptionally be disposed along the longitudinal axis 150 of the probe 100.It is particularly advantageous as the longitudinal axis 150 may be usedas a reference for the disposition measurements—combining themeasurements from the sensors 0, 1, 2, 3, 12, 13 during thedetermination may then be simplified as the separation is substantiallydetermined by the separation along the longitudinal axis 150.

The magnetic sensors 0, 1, 2, 3, 12, 13 may be configured to measurerelative or absolute magnetic intensity, to measure a vector and/orscalar component of a magnetic field. The sensors 0, 1, 2, 3, 12, 13 maybe of different types, or one or more sensors 0, 1, 2, 3, 12, 13 may bethe same. Each sensor 0, 1, 2, 3, 12, 13 comprises at least one (1D)magnetic detector. The positions may be predetermined by the designand/or measured after the probe is assembled. Additionally oralternatively, technical specifications provided by the sensormanufacturer may be used and/or simulation. Preferably, each sensormeasures a vector or 3D magnetic field—this provides the most data todetermine (or to reconstruct) the marker's disposition.

Preferably each magnetic sensor 0, 1, 2, 3, 12, 13 is a devicecomprising two (2D), three (3D or 3-axis) or more magnetic detectors,such as an IC comprising three (3D) substantially mutually perpendiculardetectors, providing measurement of three degrees of freedom atapproximately the same physical position in the probe. Again, thesepositions may be predetermined by the design and/or measured after theprobe is (partially) assembled. Additionally or alternatively, technicalspecifications provided by the sensor manufacturer may be used and/orsimulation.

In this disclosure, a sensor and detector are often usedinterchangeably. A sensor is typically a single encapsulated packagecomprising one or more detectors. If a sensor package comprises twodetectors with a physical separation between the detectors sufficientlylarge to be considered a minor sensor separation, then in the terms ofthis disclosure, such a sensor package comprises two sensors—each of thedetectors provides a measurement of a magnetic field property of themarker 200 relating to a different position within the probe 100. Aminor sensor separation is in the order of magnitude of the longitudinallength of the magnetic marker 200 to be localized, preferably 0.5 to 3times this marker length. If the physical separation between thedetectors is too small to be considered a minor sensor separation, thenin the terms of this disclosure, such a sensor package comprises onesensor—each of the detectors provides a measurement of a magnetic fieldproperty of the magnetic marker 200 relating to the same position withinthe probe 100.

These detectors may be any suitable type, such as magnetometers, fluxgate sensors, geomagnetic sensors, Lorentz force digital MEMS,magneto-inductive sensors, magneto-resistive sensors, Hall sensors,magnetic tunnel junctions and any combination thereof. Many IC packagesare available which are small and contain 3 axis detection. So a‘many-axis’ solution may be provided with simple PCB design andpreferably a smaller probe diameter. The sensor packages proposed beloware examples. They are digital and therefore relatively straightforwardto interface as less analog design is required.

TI DRV425 Flux Gate Sensor (1D)

-   -   Technology: Flux gate    -   Size: 4×4×0.8 mm    -   Range: +/−2 mT (single axis)    -   Resolution: (analogue, depends on ADC)    -   RMS noise: 0.42 uT @ 1000 Hz (0.2 uT @ 50 Hz)    -   Offset: 8.3 uT, +1.4 uT hysteresis+0.4 temperature drift    -   Gain error: 0.3%    -   Abs Max Field: >2 T in any direction    -   Note: The offset may be reduced by using a correction sensor        with a good zero-field offset performance. Another type of        sensor, for example, may be integrated in the probe 100 provide        a degree of offset and/or drift correction for the fluxgates.        Preferably, such a correction sensor is located close to, or at,        the proximal end to reduce the influence of a magnetic field        property of the magnetic marker 200.

Bosch BMM150 3-Axis Digital Geomagnetic Sensor (3D)

-   -   Technology: FlipCore    -   Size: 1.56×1.56×0.6 mm    -   Range: +/−1.2 mT (x,y); +/−2 mT (z)    -   Resolution: 0.3 uT (LSB)    -   RMS noise: 0.3 uT @ 20 samples/s    -   Offset: 40 uT without Software compensation, 2 uT after        compensation (typical)    -   Gain error: 5% (after compensation)    -   Abs Max Field: >7 T in any direction

ST LIS3MDL (1D)

-   -   Technology: Lorentz force digital MEMS    -   Size: 2×2×1 mm    -   Range: +/−1.6 mT (x,y,z) (user selectable 0.4, 0.8, 1.2 mT)    -   Resolution: 0.015 uT (LSB) (@0.4 mT range; 0.06 uT @ 1.6 mT        range)    -   RMS noise: 0.3 uT(x,y); 0.4 uT(z) @ 1.2 mT range    -   Offset: 100 uT; drifts when fields >5 mT applied    -   Gain error: 0.15% Full Scale (best fit straight-line        non-linearity)    -   Abs Max Field: <0.1 T in any direction

ST IIS2MDC (3D)

-   -   Technology: 3-axis digital output magnetometer high-accuracy,        ultra-low power    -   Noise: 0.3 uT with low-pass filter or offset cancellation        enabled. 1 SD at 20 samples per second.    -   Offset error: 6 uT; correctable to 1.2 uT over 20 degree C.        range. Hysteresis measured at 3 T was 53 uT and 13 uT with a 5        mT field.    -   Offset change: with temperature 0.03 uT per degrees C.    -   Gain error: 1.5% (typical), 7% (max)    -   Gain change: with temperature 0.03% per degrees C.

Melexis MLX90393 Micropower Triaxis Magnetometer (3D)

-   -   Technology: Hall    -   Size: 3×3×1 mm    -   Range: +/−5-50 mT (x,y,z) (user selectable)    -   Resolution: 0.16 uT(x,y); 0.3 uT(z) (LSB)    -   RMS noise: 0.7 uT(x,y); 0.9 uT(z) @ 50 Sample/s    -   Offset: 0 uT (?) 2.7 uT/C temperature drift (on-chip        compensation available)    -   Gain error: <1% cross axis sensitivity+3% over temperature    -   Abs Max Field: -

MEMSIC MMC3416xPJ (3D)

-   -   Technology: AMR    -   Size: 1.6×1.6×0.6 mm    -   Range: +/−1.6 mT (x,y,z) (user selectable 0.4, 0.8, 1.2 mT)    -   Resolution: 0.015 uT (LSB) (@0.4 mT range; 0.06 uT @ 1.6 mT        range)    -   RMS noise: 0.15 uT @ 125 samples/s    -   Offset: Repeatability Error 0.1% Full scale=1.6 uT    -   Gain error: -    -   Abs Max Field: 1 T

AKM AK09970N (3D)

-   -   Technology: HALL    -   Size: 3×3×0.6 mm    -   Range: +/−36 mT (x,y); +/−102 mT (z)    -   Resolution: 1.1 uT (LSB)    -   RMS noise: 5 uT @ 100 samples/s    -   Offset: 743 uT (x,y), 1050 uT (z)    -   Gain error: 10%    -   Abs Max Field: -

PNI RM3100 Sensor System (3D)

-   -   Technology: Magneto-inductive    -   Size: 15.24×12.8×3×10.5 mm    -   Range: +/−800 uT(z)    -   Resolution: 13 nT (LSB)    -   RMS noise: 15 nT @ 100 samples/s    -   Offset: Repeatability 8 nT hysteresis 15 nT    -   Gain error: linearity 0.5%    -   Abs Max Field: -    -   Note: Sensor system contains 3 coils and a driver IC with        digital interface

Longitudinal sensor array lengths 400 of 40 mm to 50 mm are preferred.

Each sensor 0, 1, 2, 3, 12, 13 comprises one or more detectors andmeasures respectively one or more magnetic properties of the magneticmarker 200. These property measurements are provided to a softwarealgorithm which combines them, together with physical parameters such asorientation, sensitivity, sensor separation distance, to determine theone or more dispositions of the magnetic marker 200 relative to thesensor position within the probe 100.

In some applications, triangulation principles may provide sufficientaccuracy to determine one or more dispositions of the marker 200. Eachsensor 0, 1, 2, 3, 12, 13 measurement may be converted to a dispositionto the magnetic marker 200, by considering the marker to be at a pointon a sphere of radius r from each sensor. With two sensors 0, 1, 2, 3,12, 13, the magnetic marker 200 may be anywhere along a circle where thetwo spheres intersect. In practice, the presence of noise may cause thecircular intersection to be a ring-like volume. With three sensors, thethree spheres may intersect at a point (or approximately at a point)—insome applications, this may be sufficiently accurate enough for markerlocalization.

Preferably, a 3D model of the marker magnetic field is pre-determined bysimulation and/or measurement. The sensor measurements of magneticproperties combined with the physical properties/locations, are thenfitted to the 3D model, using, for example, one or more curve fittingalgorithms, to determine the one or more dispositions. This 3D-modelapproach is advantageous as higher degrees of disposition determinationmay be provided.

To explain the basic principle, FIG. 5 depicts a finite element model ofthe magnetic field of a magnetic marker 200 and how measurements may beused to determine one or more dispositions. As depicted, the magneticfield measured 601 in T varies with distance in mm 602 from the magneticmarker 200. In this case, the finite element model is calculated for acylindrical magnet marker 200 with diameter 1.45 mm, length 2.19 mm andremanence Br of 1.43 T.

Two curve examples are depicted: the distance along the longitudinalaxis (length) 610 of the magnetic marker 200, and the distance along theradial axis (radius) 620 of the magnetic marker 200. As the orientationof the magnetic marker 200 is usually not predetermined, the probe 100is preferably configured to fit the sensor measurement data to bothmodels—for example, in this illustration, the probe 100 is configured tofit both curves to determine a disposition to the marker 200.

As depicted, the magnetic field strength 610, 615 drops rapidly as 1/r³(the inverse cube law).

One of the insights on which the invention is based is that it isadvantageous to vary the separation of the magnetic sensors—at the steeppart of the curve 610, 615 where the distal end 160 is close to themagnetic marker 200, sensors may be provided with one or more minorsensor separations—these are indicated as being comprised in the firstgroup 110 and benefit from the expected better Signal-to-Noise Ratio(SNR). At the flatter part of the curve 610, 615 where the distal end160 is further away from the magnetic marker 200, sensors may beprovided with one or more major sensor separation—these are indicated asbeing comprised in the second group 120, 130—the greater sensorseparation may provide a better Signal-to-Noise Ratio (SNR) than whenthe same sensor separation is used at all positions.

By using the sensor measurements, with additional parameters such assensor separation, sensitivity and orientation, the measurements may befitted to one or more curves 610, 615, thereby fitting the measurementdata to the 3D-model. Once an acceptable correlation is achieved, thedistance (disposition) between the probe 200 and the magnetic marker 200may be estimated—preferably both a longitudinal and a transversedisposition are determined.

The 3D-models, and any curve 610, 615, may be established frommanufacturers' technical data, from simulation, from measurement or anycombination thereof.

As depicted, the magnetic field strength 610, 615 for this particularmagnetic marker 200, becomes weaker than the Earth's magnetic field 620at a distance of 30 mm from the marker 200. To improve detectionaccuracy at comparable distances, the sensor measurements may beadvantageously compensated for any background magnetic field, such asthe Earth's magnetic field 620.

The ratio of the major sensor separation to the minor sensor separationis in the range 1.25 to 40, preferably in the range 1.6 to 7.6. Thisprovides a high degree of measurement accuracy when the distal end 160of the probe 200 is close (20 mm or less) from the magnetic marker 200and when the distal end 160 is further away (30 mm or more).

It may further be advantageous to predetermine the minor and/or majorsensor separation by considering the inverse cube law for the magneticmarker 200 being localized. This may improve the accuracy and speed ofthe curve 610, 615 fitting process.

In a second example, a 3D model of a magnetic field of a differentmagnetic marker is pre-determined by simulation and/or measurement. Thesensor measurements of magnetic properties combined with the physicalproperties/locations, are then fitted to the 3D model, using, forexample, one or more curve fitting algorithm depicted in FIG. 6. Afinite element model of the magnetic field of a different magneticmarker 200 is depicted. The magnetic field measured 701 in T varies withdistance in mm 702 from the magnetic marker 200. In this case, thefinite element model is calculated for a cylindrical magnet marker 200with diameter 1.75 mm, length 5 mm and remanence Br of 1.43 T.

Two curve examples are again depicted: the distance along thelongitudinal axis (length) 710 of the magnetic marker 200, and thedistance along the radial axis (radius) 720 of the magnetic marker 200.As the orientation of the magnetic marker 200 is usually notpredetermined, the probe 100 is preferably configured to fit the sensormeasurement data of both models—for example, in this illustration, theprobe 100 is configured to fit both curves to determine a disposition tothe marker 200.

As depicted, the magnetic field strength 710, 715 also drops rapidly as1/r³ (the inverse cube law)

Similar to FIG. 5, the separation of the magnetic sensors is varied—atthe steep part of the curve 710, 715 where the distal end 160 is closeto the magnetic marker 200, sensors may be provided with one or moreminor sensor separations—these are indicated as being comprised in thefirst group 110. At the flatter part of the curve 710, 715 where thedistal end 160 is further away from the magnetic marker 200, sensors maybe provided with one or more major sensor separation—these are indicatedas being comprised in the second group 120, 130.

The 3D models and any curves 710, 715 may be established frommanufacturers' technical data, from simulation, from measurement or anycombination thereof.

As depicted, the magnetic field strength 710, 715 for this particularmagnetic marker 200, becomes weaker than the Earth's magnetic field 720at a distance of 40 mm from the marker 200. To improve detectionaccuracy at comparable distances, the sensor measurements may beadvantageously compensated for any background magnetic field, such asthe Earth's magnetic field 720.

The ratio of the major sensor separation to the minor sensor separationis again in the range 1.25 to 40, preferably in the range 1.6 to 7.6.This again provides a high degree of measurement accuracy when thedistal end 160 of the probe 200 is close (20 mm or less) from themagnetic marker 200 and when the distal end 160 is further away (30 mmor more).

As mentioned for FIG. 5, it may further be advantageous to predeterminethe minor and/or major sensor separation by considering the inverse cubelaw for the magnetic marker 200 being localized.

FIG. 2A depicts a further embodiment 101 of the probe—for clarity, themagnetic marker 200 being localized and the skin 300 are not depicted.However, the measurement situation is analogous to the situationdepicted in FIG. 1A.

The probe 101 comprises a distal end 160, configured and arranged to bedisposed close to an outer surface of skin 300. The probe 101 furthercomprises three rows of two or more magnetic sensors. These may beconfigured and arranged to be comprised in up to three different groupsof sensors 110, 120, 130. As described below, each sensor may beconfigured and arranged to perform a single function or to contribute toa plurality of functions. The three groups explained above withreference to FIG. 1A are also present, although not all rows comprisesensors in each group. FIG. 2B depicts the reference number assigned toeach magnetic sensor for ease of reference—in this embodiment, a minimumof six magnetic sensors are provided, and typically sixteen sensors areused in a 2D array. The sensors are grouped as follows:

-   -   110: a first group of at least two magnetic sensors, close to        the distal end 160 of the probe 101. As depicted in FIG. 2B,        these may be sensors 0, 1, 2, 4, 5, 6, 8, 9 and 10 in a 3×3 2D        array. Not all sensors positions need to be occupied in        practice—one or more sensors in the 3×3 2D array may be        physically omitted or disabled in software. These sensors are        configured and arranged such that adjacent sensors are separated        from each other by a minor sensor separation in both        longitudinal and transverse directions.

However, by combining measurements from non-adjacent sensors, one ormore sensors in the first group 110 may be considered to be separated bya major sensor separation in a transverse direction. For example, thefollowing are separated transversely by a major sensor separation:

-   -   sensors 2 and 10;    -   sensors 1 and 9;    -   sensors 0 and 8;

Similarly, the following are separated longitudinally by a major sensorseparation:

-   -   sensors 2 and 1;    -   sensors 6 and 4;    -   sensors 10 and 8;    -   120: a second group of at least one magnetic sensor. As depicted        in FIG. 2B, this may be sensors 3, 12, 7, 11 and 14 in a 2×3 2D        array. Not all sensors positions need to be occupied in        practice—one or more sensors in the sx3 2D array may be        physically omitted or disabled in software—in FIG. 2B, for        example, there is no sensor between 12 and 14. These sensors 120        are closer to a proximal end 165 of the probe 101 than the first        group 110 (in other words, further away from the distal end        160). In this embodiment, a magnetic sensor from the second        group 120 is longitudinally separated from the first group 110        by a major sensor separation. The major sensor separation is        again greater than the minor sensor separation. For example, the        following are separated longitudinally by a major sensor        separation:    -   sensors 3 and 0, 1 or 2;    -   sensors 7 and 4, 5 or 6;    -   sensors 11 and 8, 9 or 10;    -   sensors 12 and 0, 1, or 2;    -   sensors 12 and 3;    -   sensors 14 and 8, 9, or 10;    -   sensors 14 and 11;

In addition, one or more sensors in the second group 120 may beseparated by a major sensor separation in a transverse direction. Forexample, the following are separated transversely by a major sensorseparation:

-   -   sensors 3 and 11;    -   sensors 12 and 14;    -   130: optionally, a third group of at least one magnetic sensor        is provided. As depicted in FIG. 2B, this may be sensors 13 and        15 in a 1×3 1D array. Not all sensors positions need to be        occupied in practice—one or more sensors in the 1×3 2D array may        be physically omitted or disabled in software—in FIG. 2B, for        example, there is no sensor between 13 and 15. These sensors 130        are closer to the proximal end 165 of the probe 101 than the        second group 120 (in other words, further away from the distal        end 160). They may be configured and arranged:    -   as a compensation sensor, as described above in reference to        FIG. 1A.    -   alternatively or additionally, magnetic sensors in the third        group 130 may be configured and arranged to provide measurements        to determine one or more dispositions (distance) when the distal        end 160 of the probe 101 is further away from the magnetic        marker 200. For example, the following are separated by a major        sensor separation:    -   sensors 13 and 0, 1, or 2;    -   sensors 15 and 8, 9, or 10;    -   For example, the following may be separated by a major sensor        separation in a longitudinal direction:    -   sensors 13 and 12 or 3;    -   sensors 15 and 14 or 11;

In addition, one or more sensors in the third group 130 may be separatedby a major sensor separation in a transverse direction. For example, thefollowing are separated transversely by a major sensor separation:sensors 13 and 15.

As depicted in FIG. 2A, the center of the packages may optionally bedisposed along one of more transverse axes. Sensors 0, 4, 8 may lie on afirst transverse axis 181, sensors 1, 5, 9 on a second transverse axis182, sensors 2, 6, 10 on a third transverse axis 183, sensors 3, 7, 11on a fourth transverse axis 184, for sensor 12, 14 on a fifth transverseaxis 185, and sensor 13, 15 lies on a sixth transverse axis 186. Thetransverse axes 181 to 186 are substantially perpendicular to thelongitudinal probe axis 150. Combining the measurements from the sensorsmay then be simplified as the transverse separation is substantiallydetermined by the separation along the respective longitudinal axes 150,151, 152.

As depicted in FIG. 2A, the magnetic sensors 0, 1, 2, 3, 12, 13 mayoptionally be disposed along the longitudinal axis 150 of the probe 101.In addition, magnetic sensors 0, 1, 2, 3, 12, 13 may optionally bedisposed along a second longitudinal axis 151 of the probe 101, andmagnetic sensors 0, 1, 2, 3, 12, 13 may optionally be disposed along athird longitudinal axis 152 of the probe 101. Combining the measurementsfrom the sensors may then be simplified as the longitudinal separationis substantially determined by the separation along the respectivelongitudinal axes 150, 151, 152.

It is particularly advantageous if one of the longitudinal axes 150,151, 152 may be used as a reference for the dispositiondetermination—combining the measurements from the sensors may then besimplified as the separation is substantially determined by theseparation along the longitudinal axes 150, 151, 152 used as reference.

Simulations using arrays of the ST IIS2MDC (3D) sensors wereperformed—each of these sensor package measures three degrees of freedomusing three detectors. As the detectors are very close together,effectively measuring at a single position of the probe, each package isconsidered a magnetic sensor as described in this disclosure.

The package size is 2 mm×2 mm×0.7 mm—this means that with at least 0.5mm space between sensor packages, the minimum sensor separation isapproximately 2.5 mm. The following configurations produced satisfactoryresults:

-   -   1. Sensor layout: as depicted in FIGS. 2A and 2B with sensors        disposed along three longitudinal axes 150 to 152, and six        transverse axes 181 to 186        -   Longitudinal array length 400: approx. 40 mm        -   Transverse array width 500: approx. 10 mm        -   Minor sensor separation in group 110: approx. 5 mm        -   Distance between transverse axes 181 and 182: approx. 5 mm        -   Distance between transverse axes 182 and 183: approx. 5 mm        -   Distance between transverse axes 183 and 184: approx. 6.67            mm        -   Distance between transverse axes 184 and 185: approx. 10 mm        -   Distance between transverse axes 185 and 186: approx. 15 mm    -   2. Sensor layout: as depicted in FIGS. 2A and 2B with sensors        disposed along three longitudinal axes 150 to 152, and six        transverse axes 181 to 186        -   Longitudinal array length 400: approx. 50 mm        -   Transverse array width 500: approx. 10 mm        -   Minor sensor separation in group 110: approx. 5 mm        -   Distance between transverse axes 181 and 182: approx. 5 mm        -   Distance between transverse axes 182 and 183: approx. 5 mm        -   Distance between transverse axes 183 and 184: approx. 10 mm        -   Distance between transverse axes 184 and 185: approx. 15 mm        -   Distance between transverse axes 185 and 186: approx. 15 mm    -   3. Sensor layout: as depicted in FIGS. 2A and 2B with sensors        disposed along three longitudinal axes 150 to 152, and six        transverse axes 181 to 186        -   Longitudinal array length 400: approx. 40 mm        -   Transverse array width 500: approx. 5 mm        -   Longitudinal minor sensor separation in group 110: approx. 5            mm        -   Transverse minor sensor separation in group 110: approx.            2.55 mm        -   Distance between transverse axes 181 and 182: approx. 5 mm        -   Distance between transverse axes 182 and 183: approx. 5 mm        -   Distance between transverse axes 183 and 184: approx. 6.67            mm        -   Distance between transverse axes 184 and 185: approx. 10 mm        -   Distance between transverse axes 185 and 186: approx. 15 mm

FIGS. 7A and 7B depict a further embodiment 105 of a probe with analternative sensor layout—for clarity, the magnetic marker 200 beinglocalized and the skin 300 are not depicted. However, the measurementsituation is analogous to the situation depicted in FIG. 1A and FIG. 2A

The probe 105 comprises a distal end 160, configured and arranged to bedisposed close to an outer surface of skin 300. The probe 105 furthercomprises five rows of two or more magnetic sensors. These may beconfigured and arranged to be comprised in up to two different groups ofsensors 110, 130. As described below, each sensor may be configured andarranged to perform a single function or to contribute to a plurality offunctions. Two of the three groups explained above with reference toFIG. 1A and FIG. 2A are also present, although not all rows comprisesensors in each group. FIG. 7B depicts the reference number assigned toeach magnetic sensor for ease of reference—in this embodiment, a minimumof six magnetic sensors are provided, and typically sixteen sensors areused in a 2D array. The sensors are grouped as follows:

-   -   110: a first group of at least two magnetic sensors, close to        the distal end 160 of the probe 105. As depicted in FIG. 7B,        these may be sensors 0, 1, 2, 4, 5, 6, 8, 9, 10, 12, 13, 14 and        15 in a 5×5 2D array. Not all sensors positions need to be        occupied in practice—one or more sensors in the 5×5 2D array may        be physically omitted or disabled in software. These sensors are        configured and arranged such that adjacent sensors are separated        from each other by a minor sensor separation in both        longitudinal and transverse directions. Adjacent sensors are        diagonally adjacent to each other.

However, by combining measurements from non-adjacent sensors, one ormore sensors in the first group 110 may be considered to be separated bya major sensor separation in a transverse direction. For example, thefollowing are separated transversely by a major sensor separation:

-   -   sensors 3 and 7, 7 and 11, 2 and 6, 6 and 10, 1 and 5, 5 and 9,        0 and 4, 4 and 8;    -   sensors 13 and 15, 12 and 14;

Similarly, the following are separated longitudinally by a major sensorseparation:

-   -   sensors 0 and 1, 1 and 2, 2 and 3, 4 and 5, 5 and 6, 6 and 7, 8        and 9, 9 and 10, 10 and 11;    -   sensors 14 and 15, 12 and 13    -   130: optionally, a third group of at least one magnetic sensor        is provided, as described above with reference to FIG. 1A and        FIG. 2A.

Simulations using arrays of the ST IIS2MDC (3D) sensors were alsoperformed with these alternative layouts:

-   -   4. Sensor layout: as depicted in FIGS. 7A and 7B with sensors        disposed along five longitudinal axes 150 to 154, and six        transverse axes 181 to 186.        -   Note: satisfactory results were obtained from this shorter            array after subtracting the background field measured using            one or more sensors in group 130.        -   Longitudinal array length 400: approx. 15 mm        -   Transverse array width 500: approx. 10 mm        -   Minor sensor separation in group 110: approx. 3.5 mm        -   Distance between transverse axes 181 and 182: approx. 3.5 mm        -   Distance between transverse axes 182 and 183: approx. 3.5 mm        -   Distance between transverse axes 183 and 184: approx. 3.5 mm        -   Distance between transverse axes 184 and 185: approx. 3.5 mm        -   Distance between transverse axes 185 and 186: approx. 7 mm

FIGS. 8A and 8B depict a further embodiment 106 of a probe with analternative sensor layout—for clarity, the magnetic marker 200 beinglocalized and the skin 300 are not depicted. However, the measurementsituation is analogous to the situation depicted in FIG. 1A, FIG. 2A andFIG. 7A

The probe 105 comprises a distal end 160, configured and arranged to bedisposed close to an outer surface of skin 300. The probe 106 furthercomprises five rows of two or more magnetic sensors. These may beconfigured and arranged to be comprised in up three different groups ofsensors 110, 120, 130. As described below, each sensor may be configuredand arranged to perform a single function or to contribute to aplurality of functions. The three groups explained above with referenceto FIG. 1A and FIG. 2A are also present, although not all rows comprisesensors in each group. FIG. 8B depicts the reference number assigned toeach magnetic sensor for ease of reference—in this embodiment, a minimumof six magnetic sensors are provided, and typically sixteen sensors areused in a 2D array. The sensors are grouped as follows:

-   -   110: a first group of at least two magnetic sensors, close to        the distal end 160 of the probe 106. As depicted in FIG. 8B,        these may be sensors 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 in        a 5×5 2D array. Not all sensors positions need to be occupied in        practice—one or more sensors in the 5×5 2D array may be        physically omitted or disabled in software. These sensors are        configured and arranged such that adjacent sensors are separated        from each other by a minor sensor separation in both        longitudinal and transverse directions. Adjacent sensors are        diagonally adjacent to each other.

However, by combining measurements from non-adjacent sensors, one ormore sensors in the first group 110 may be considered to be separated bya major sensor separation in a transverse direction. For example, thefollowing are separated transversely by a major sensor separation:

-   -   sensors 2 and 5, 5 and 8, 1 and 4, 4 and 7, 0 and 3, 3 and 6;    -   sensors 10 and 12, 9 and 11;

Similarly, the following are separated longitudinally by a major sensorseparation:

-   -   sensors 0 and 1, 1 and 2, 3 and 4, 4 and 5, 6 and 7, 7 and 8;    -   sensors 9 and 10, 11 and 12    -   sensors 13 and 5;    -   sensors 14 and 15, 14 and 13    -   130: optionally, a third group of at least one magnetic sensor        is provided, as described above with reference to FIG. 1A and        FIG. 2A.

Simulations using arrays of the ST IIS2MDC (3D) sensors were alsoperformed with these alternative layouts:

-   -   5. Sensor layout: as depicted in FIGS. 8A and 8B with sensors        disposed along five longitudinal axes 150 to 154, and eight        transverse axes 181 to 188.        -   Longitudinal array length 400: approx. 40 mm        -   Transverse array width 500: approx. 10 mm        -   Minor sensor separation in group 110: approx. 5 mm        -   Distance between transverse axes 181 and 182: approx. 5 mm        -   Distance between transverse axes 182 and 183: approx. 5 mm        -   Distance between transverse axes 183 and 184: approx. 5 mm        -   Distance between transverse axes 184 and 185: approx. 5 mm        -   Distance between transverse axes 185 and 186: approx. 5 mm        -   Distance between transverse axes 186 and 187: approx. 10 mm        -   Distance between transverse axes 187 and 188: approx. 15 mm

Some conventional detectors and probes require a high remanence (Br) forthe magnetic markers 200 due to their low sensitivity—the probeaccording to the invention allows a higher sensitivity of magneticmarker detection, allowing a broader range of markers to be localized atboth close distances and further away.

The embodiments described above comprises a substantiallytwo-dimensional (2D) array of magnetic sensors. FIGS. 3A, 3B and 3Cdepict three-dimensional arrangements (3D arrays) of magnetic sensors.

FIG. 3A depicts a further embodiment 102 of the probe—for clarity, themagnetic marker 200 being localized and the skin 300 are not depicted.However, the measurement situation is analogous to the situationdepicted in FIG. 1A and FIG. 2A.

The probe 102 comprises three rows of two or more magnetic sensors, thecenter of the packages being optionally disposed along six transverseaxes 181 to 186. The transverse axes 181 to 186 are substantiallyperpendicular to three longitudinal probe axis 150 to 152. The sensorsmay be optionally disposed along these longitudinal axis 150 to 152,with each row of sensors being disposed on a separate face oflongitudinally extended cube. The shape may also be described as asquare or rectangular prism. The transverse cross-section of the probe102 is a square or rectangle. The probe 102 of FIG. 3A may be formed byproviding the probe 101 of FIG. 2A on a flexible substrate, and foldingit between the rows of magnetic sensors.

Additionally, a further row of sensors may be disposed on the fourthface of the longitudinally-extended probe.

FIG. 3B depicts a further embodiment 103 of the probe. The probe 103comprises three rows of two or more magnetic sensors, the center of thepackages being optionally disposed along six transverse axes 181 to 186.The transverse axes 181 to 186 are substantially perpendicular to threelongitudinal probe axis 150 to 152. The sensors may be optionallydisposed along these longitudinal axis 150 to 152, with each row ofsensors being disposed on a separate face of longitudinally-extendedtriangular prism. The transverse cross-section of the probe 102 is atriangle. The probe 103 of FIG. 3B may be formed by providing the probe101 of FIG. 2A on a flexible substrate, and folding it between the rowsof magnetic sensors.

The skilled person will also realize that other shapes of transversecross-section may be used in longitudinally-extended forms, such as acircle, an oval, an ellipse irregular triangle, or a trapezoid.

FIG. 3C depicts a further embodiment 104 of the probe. The probe 103comprises two rows of two or more magnetic sensors, the center of thepackages being optionally disposed along six transverse axes 181 to 186.The transverse axes 181 to 186 are substantially perpendicular to twolongitudinal probe axis 150 and 151. The sensors may be optionallydisposed along these longitudinal axis 150 and 151, with each row ofsensors being disposed on a separate face of longitudinally-extendedwedge (or L) shape. The transverse cross-section of the probe 104 is acorner. The probe 104 of FIG. 3C may be formed by using two rows ofmagnetic sensors on a flexible substrate, and folding it between therows of magnetic sensors.

The skilled person will also realize that the two faces may be bent,such that they contact each other on the inside. This provides a flatwand very suitable for marker 200 localization.

The skilled person will realize from the embodiments depicted in FIGS.3A, 3B and 3C that a 3-dimensional sensor array may be provided bymounted sensors on a rigid PCB having the correct shape, or using aflexible substrate

Note that a 3-dimensional arrangement of sensors may provide a higherdegree of sensitivity in a plurality of directions.

By suitable dimensioning and/or substrate/component selection, the probemay have an approximately cylindrical shape, such as a probe, wand orpencil. If the probe is configured to be inserted through the skin or ina cavity, a small diameter may be preferred. With such procedures,lateral (or transverse) measurement may be less critical, and may evenbe omitted from the design by having a single row of sensors, extendingalong the probe longitudinal axis 150 as depicted in FIGS. 2A and 2B.

Determining the one or more dispositions of the magnetic marker 200using the measurement data from the magnetic sensors may be performed inany suitable way. For example, three types of algorithms were evaluated:

(1) a solver which uses iteration to solve for the unknown parameters;

(2) a rangefinder which gives an indication of the distance and possiblydirection of the magnetic marker 200; and

(3) differential measurement

(1) The iterative localization algorithm finds the position of themagnet by simulating the expected magnetic fields at the known sensorpositions by assuming a magnet 200 (with a known dipole moment) at someposition and orientation. For these marker locations and orientations, aforward 3D-model (as described above) may be provided to simulate thepredicted field at each magnetic sensor. The optimization algorithm maythen minimize the difference between the measured and predicted fieldsat the sensor. The sum of squares (least squares) may be minimized, orany other suitable approach.

It then compares the simulated magnetic fields against the measurementsand, using optimization techniques, iteratively finds the set ofparameters that minimizes the difference between the measurement andprediction.

The optimization may fit for 9 parameters:

-   -   Magnet position (x, y, z)    -   Magnet orientation (nx, ny, nz)    -   Background field (B0x, B0y, B0z)

The magnet orientation is a unit vector and the cost function appliesthis constraint internally, which means that there are 8 degrees offreedom. Optimization may use any convenient algorithm, such the TrustRegion Reflective algorithm in least_squares from the scipy.optimizepackage. This particular algorithm is reasonably efficient and allowsoptimization of the parameters within given bounds. Many of theconventional magnetic marker localization publications suggest theLevenberg-Marquardt algorithm, but this is less preferred because it isunbounded. When applied to the probe according to the invention,unbounded optimization occasionally gave unrealistic solutions while thebounded Trust Region Reflective algorithm gave more robust solutions.

The algorithm evaluates and returns the Jacobian, which may be used toindicate uncertainty in the estimated position. This is a modifiedversion of the approach used to indicate uncertainty in a GPS positionfix. The uncertainty provides an indication of whether the magneticmarker 200 is inferred to be inside the beam or so poorly localized thatit is deemed not detected.

A position boundary of ±80 mm was assumed in all directions. Thebackground field vector was limited to ±100 μT in each direction. Theorientation of the magnetic field was constrained to ±1 in eachdirection, and its magnitude was constrained by the cost function to 1.

The optimization algorithm used was a gradient method and the Jacobianwas calculated numerically. When the probe is stationary, the markerposition inferred may jump to another location—to stabilize this, thesolution from the previous sample may be fed back as the initialstarting point for the next sample.

(2) The non-iterative localization algorithm (rangefinder) just infersthe distance to the magnetic marker 200. The distance to the marker 200from each sensor may be estimated from the magnitude of the magneticfield measured by each sensor, which is given by:

${B} = {{m}\frac{ \sqrt{}( {1 + {3\cos^{2}\theta}} ) }{r^{3}}}$

But |B| depends on the orientation of the magnet 200; θ is the anglebetween the magnet's pole and the vector between the magnet 200 and thesensor. This means that at a given distance r, |B| is twice as strongwhen the sensor is along the poles of the magnet (i.e. inclination angleθ=0 in spherical polar coordinates relative to the dipole) than when thesensor is around the ‘equator of the magnet θ=90 degrees). |B|∝1/r³ sothe error in r without knowledge of θ is 1/(³√2)=0.79.

(3) FIG. 4 depicts an example of differential measurement versus lateraldisplacement characteristic which may be used to convert the L-R signalfrom a sensor to lateral displacement. From each sensor (L and R) themagnitude of the magnetic field is measured. Differential measurementsare made by comparing field strengths at neighboring sensors. Forexample, left versus right; front versus back, top versus bottom. If thedifferential is zero, the marker 200 is disposed close to the middlepoint between the sensors. If the differential is positive, the markeris disposed more to the right. If the differential is negative, themarker is disposed more to the left

The x-axis shows displacement X in centimeters, from −3.0 to +3.0. TheY-axis shows the L-R signal from −0.60 to 0.60. Using a magnetic marker200 which is cylindrical, made of NdFeB, 4 mm in length, and 2 mm indiameter, the L-R signal was measured at transverse dispositions ofX=−2.0, −1.0, 0, +1.0 and +2.0. These distances are in the range five totwenty times a dimension of the magnetic marker 200. At X=0, themagnetic marker 200 is disposed on the probe longitudinal axis 150.Based on these values, a characteristic has been fitted, which is astraight line from −2.5, −0.52775 to 2.5, 0.52775. In other words, thedistance X may be calculated from L−R=0.2111X. In this example, thecorrelation factor (R²) of the linear curve fit is 0.9328.

For any of the approaches, it may be advantageous to filter the magneticfield measurements before the optimization. It may also be advantageousto filter the predicted dispositions.

Additionally, it may be advantageous to weigh the sensors by theSignal-to-Noise Ratio (SNR) and/or weigh sensors based on their relativepositions within the probe 100. For example, give the sensors closest tothe proximal end 165 a lower weighting as they may measure a weakermagnetic signal than sensors closest to the distal end 160.

Additionally, it may also be advantageous to constrain the search spaceby excluding magnetic marker 200 locations that are unlikely, or evenimpossible, such as those inside the probe.

Alternatively, an approach such as the Unscented Kalman Filter toconstrain the location using previously deduced locations of the probemay be used. This is described further in Marius Birsan, “UnscentedParticle Filter for Tracking a Magnetic Dipole Target”, Proceedings ofOCEANS 2005 MTS/IEEE (2005).

Alternatively, instead of estimating a unique location, it may beadvantageous to estimate the probability distribution of the markerlocation, which will be a 3-dimensional region in space.

One of the insights on which the invention is based is that known probeswere limited in that only a relative distance is measured, and thesensitivity is optimized for dispositions where the distal end isrelatively close to the magnetic marker 200. At distances further away,the sensitivity and accuracy with conventional probes is reduced,increasing the chance of incorrect detection. By providing at least onemeasurement using sensors having a major sensor separation, a probe isprovided with sensitivity both close to the magnetic marker 200 andfurther away.

The higher degree of accuracy and sensitivity allows improved guidanceto be provided to the user using, for example, visual, audio orvibrational cues and/or visual information. Conventional probes useAC-susceptometry and provide no 3D guidance and no directionality, onlyrelative proximity—the prior art systems rely on the healthcareprofessional guessing the position of the marker, and moving and tiltingthe probe to see the positions and orientations with the highestmeasurement value. This often results in a trial and error way ofdetecting the disposition of magnetic markers. Other probes use RFID orElectro-Magnetic, but this only provides (relative) distance, and not 3Dinformation. Some probes use radioactive seed—this may provide a degreeof direction (collimation), but no disposition information is providedas there is a low degree of signal decay with distance.

A suitable user interface may be provided to give the user guidance andcues, either comprised in the probe 100 or comprised in the magneticdetector. This may be relatively simple, for example, audio feedback torepresent a longitudinal disposition 250, 255 (e.g. by modifying pitch),audio feedback to represent a transverse disposition 260. (e.g. by acontinuous tone when the position of the magnetic marker 200 coincideswith the probe longitudinal axis 150. A more complicated user interfacemay also be provided, such as a graphical representation (e.g. 2Dtargeting cross) indicating direction, or a graphical 3D representationshowing the marker 200 and the relative location of the probe 100 (orvice-versa).

This lack of clear guidance is also the case for conventionalradio-active markers, that have the additional disadvantage of safetyissues in addition to lacking 3D guidance. For conventionalElectro-Magnetic and RFID markers, they have the disadvantages that theymust remain active, they are susceptible to failure, they are bulky inaddition to lacking 3D guidance.

The magnetic marker 200 may comprise any suitable magnetic materialssuch as AlNiCo, SmCo, NdFeB and any combination thereof. For example:cylindrical, made of NdFeB, 4 mm in length, and 2 mm in diameter.

Preferably, magnetic markers 200 should be used with a small size andhigh remnant field. The probe 100 may be configured and arranged todetect the disposition of a marker 200 at several centimeters distance250, 255, 260. The probe 100 may be dimensioned for convenient handhelduse.

Such magnetic markers are almost unbreakable, and have passive materialproperties. They are biologically inert, making them inherently suitablefor implantation. In addition, health risks and regulations are reducedwhen compared to radio-active markers. Also the availability and supplyof radio-active markers is limited. Also, there are no active componentswhich may fail during use, such as with Electro-Magnetic and RFID types.

MagSeed® magnetic marker 200 are advantageous to be used with the probeaccording to the invention—they are available in lengths less than 5 mm,which is smaller than typical lesions and tumors. They have a highmagnetic susceptibility, and typically have a minimal remnant field >0.3T of the marker's magnetic material, allowing a detection distance ofseveral centimeters. In addition, this degree of detection is alsopossible against the background field of the Earth, which is typicallyapproximately 10 uT. Conventional detectors for MagSeed® magneticmarkers rely on susceptibility detection, which is limited in detectiondistance in addition to lacking 3D guidance.

3 mm lengths are also available—this may be used for surgicalapplications. With a suitably configured and arranged probe 100, thedisposition of these 3 mm markers 200 after implantation may bedetermined at both several centimeter distance of the distal end 160from the marker 200 (using the third and fourth magnetic sensors), andalso may be determined at close (approximately) 1-2 mm distances (usingthe first and second magnetic sensors).

To be configured for implantation, the magnetic markers are preferablyencapsulated or packaged in a biologically-safe material, such astitanium, parylene, silicone or any combination thereof.

The skilled person will realize that with a suitable translation formulaor a matrix, marker 200 dispositions relative to a reference axis, suchas the transverse and longitudinal axes, may be converted todispositions relative to the probe longitudinal axis 150 and anytransverse axis passing through a surface of the distal end 160 of theprobe. Vice-versa conversions may also be performed where necessary.

The probe according to the invention may comprise any suitable magneticsensor, and any suitable mix of sensors.

For example: using a magnetic marker 200 which is cylindrical, made ofNdFeB, 4 mm in length, and 2 mm in diameter, the following measurementvalues were obtained with a probe 101 configuration as depicted in FIGS.2A and 2B.

-   -   6. Sensor layout: as depicted in FIGS. 2A and 2B with sensors        disposed along three longitudinal axes 150 to 152, and six        transverse axes 181 to 186        -   Longitudinal array length 400: approx. 40 mm        -   Transverse array width 500: approx. 10 mm        -   Minor sensor separation in group 110: approx. 5 mm        -   Distance between transverse axes 181 and 182: approx. 5 mm        -   Distance between transverse axes 182 and 183: approx. 5 mm        -   Distance between transverse axes 183 and 184: approx. 7 mm        -   Distance between transverse axes 184 and 185: approx. 10 mm        -   Distance between transverse axes 185 and 186: approx. 15 mm

The distal end 160 of the probe 101 was disposed approximately 20 mmlongitudinally from the magnetic marker, and disposed approximately −20mm transversely from magnetic marker (in other words, the magneticmarker 200 was lying “below” the probe longitudinal axis 150 as depictedin FIG. 1A). The probe for these measurements comprised Melexis MLX90393Micropower Triaxis Magnetometer sensors (see above).

[Marker] 186 185 184 183 182 181 150 3.3 6.4 10.3 17.7 14.8 26.9 15112.4 19.9 26.1 38.3 152 3.9 7.8 12.3 21.5 38.5 50.2

Using the same sensor layout, a similar measurement was performed withthe distal end 160 of the probe 101 disposed approximately 15 mmlongitudinally from the magnetic marker, and disposed approximately +10mm transversely from magnetic marker (in other words, the magneticmarker 200 lying “above” the probe longitudinal axis 150, the oppositesituation to that depicted in FIG. 1A).

[Marker] 186 185 184 183 182 181 150 6.2 12.1 25.9 51.4 89.9 164.5 15124.9 47.3 77.8 142.6 152 5.5 11.6 22.2 41.4 57.6 112.4

The magnetic sensor array may be mounted as a 2-dimensional array on aPCB using conventional techniques. The array may also be mounted on aflat large-dimensioned substrate if the probe is used for transcutaneouslocation only. For example, locating the magnetic marker 200 prior to asurgeon making an incision.

Having a large array of sensors may be advantageous as individualsensors may be selected and deselected to create a “scanning” effect. Inother words, the disposition of the marker may be determined with no (orminimal) movement of the probe in transverse or longitudinal directions.With such procedures, depth (or longitudinal disposition determination)may be less critical, or may even be omitted from the design.

Additionally, the probe may comprise additional sensors to provide formeasurement of the orientation of probe. For example, the pitch, rolland yaw angle of the probe from an IMU (inertial measurement unit)sensor, the orientation relative to the background magnetic field fromthe background field sensor or other inputs. This orientation may alsobe considered when determining the disposition of the magnetic marker200.

Any other inputs that give position information may be used—for example,an optical sensor, similar to the sensor used on an optical mouse, maybe used to determine a contact point on the surface of the skin.

Although the present invention has been described in connection withspecific exemplary embodiments, it should be understood that variouschanges, substitutions, and alterations apparent to those skilled in theart can be made to the disclosed embodiments without departing from thespirit and scope of the invention as set forth in the appended claims.

Note that the use of the labels first, second, third and fourth for thesensors in the claims is distinct and not necessarily the same as thelabels first, second and third for the sensor groups used in thedescription. The first, second, third and fourth sensors may be selectedfrom any of those groups in accordance with the functionality performedby the different embodiments.

In general, a magnetic sensor may be functionally configured andarranged to co-operate with both one or more other magnetic sensors,forming one or more magnetic sensor pairs. In some cases, it may beadvantageous for a single sensor to be configured and arranged to beseparated by a minor sensor separation with a further magnetic sensor,as well as configured and arranged to be separated by a major sensorseparation with a still further magnetic sensor. Alternatively oradditionally, the skilled person will also realize that it may beconvenient to use an axial/radial co-ordinate system for two or moremagnetic sensors. A plurality of axial axes may be used, each with oneor more corresponding radial axes—as indicated above, the skilled personcan easily convert between the different co-ordinate systems. Forexample, when magnetic sensors 110, 120, 130 are configured and arrangedto be separated by the major sensor disposition along the probelongitudinal axis, this may be considered to be an axial axis or axialdirection. This may also be considered, for example, to be a centralaxial axis. Any magnetic sensors 110, 120, 130 configured and arrangedto be separated by a minor sensor disposition may then be considered tobe disposed along a radial axis or radial direction.

REFERENCE NUMBERS USED IN DRAWINGS

-   0-15 a first to sixteenth magnetic sensor-   100 a first embodiment of a magnetic field probe-   101 a second embodiment of a magnetic field probe-   102 a third embodiment of a magnetic field probe-   103 a fourth embodiment of a magnetic field probe-   104 a fifth embodiment of a magnetic field probe-   110 first sensor group-   120 second sensor group-   130 third sensor group-   150 a first probe longitudinal axis-   151 a second longitudinal sensor axis-   152 a third longitudinal sensor axis-   160 a distal end of probe-   165 a proximal end of probe-   181 a first transverse sensor axis-   182 a second transverse sensor axis-   183 a third transverse sensor axis-   184 a fourth transverse sensor axis-   185 a fifth transverse sensor axis-   186 a sixth transverse sensor axis-   200 implantable magnetic marker-   250 longitudinal distance below outer surface of skin (depth)-   255 spacing between distal end of probe & outer surface of skin    (clearance)-   260 transverse disposition of magnetic marker from longitudinal axis-   300 an outer surface of skin-   400 longitudinal extent of the sensor array-   500 transverse extent of the sensor array-   601 magnetic field (T)-   602 distance (mm)-   610 finite element model of field along magnetic marker axis-   615 finite element model of field along magnetic marker radius-   620 Earth magnetic field-   701 magnetic field (T)-   702 distance (mm)-   710 finite element model of field along magnetic marker axis-   715 finite element model of field along magnetic marker radius-   720 Earth magnetic field

The invention claimed is:
 1. A magnetic field probe for determining adisposition of an implantable magnetic marker, the probe extending alonga probe longitudinal axis, the probe comprising: a distal end,configured and arranged to be disposed close to an outer surface ofskin; a first magnetic sensor close to the distal end; a second magneticsensor, close to the distal end, configured and arranged to be separatedby a minor sensor separation from the first magnetic sensor, the firstmagnetic sensor and the second magnetic sensor being configured andarranged to determine, in use, one or more dispositions of theimplantable magnetic marker; a third magnetic sensor close to a proximalend, configured and arranged to be separated by a major sensorseparation from the second magnetic sensor, the third and secondmagnetic sensor being configured and arranged to further determine, inuse, the one or more dispositions of the implantable magnetic marker;wherein: the major sensor separation is larger than the minor sensorseparation; a ratio of the major sensor separation to the minor sensorseparation is in the range 1.25 to 40; the first magnetic sensor and thesecond magnetic sensor are substantially disposed along a transverseaxis, the transverse axis intersecting the probe longitudinal axis; andwherein at least one of the first magnetic sensor, the second magneticsensor, and the third magnetic sensor are comprised in an arrangement oftwo or more magnetic sensors in a 1D, 2D, or 3D array, wherein the firstmagnetic sensor and the second magnetic sensor are co-linear along thetransverse axis, and wherein the second magnetic sensor and the thirdmagnetic sensor are co-linear along an axis parallel to the probelongitudinal axis, and wherein the distal end of the probe is furtherconfigured and arranged to contact an outer surface of skin and/or to beinserted through an outer surface of skin and/or to be inserted into abody cavity, and wherein the probe is configured to detect the magneticfield due to the orientation of the sensors relative to the probelongitudinal axis, and wherein a length of the probe longitudinal axisis greater than a length of the probe transverse axis.
 2. The probeaccording to claim 1, wherein the probe further comprises: a fourthmagnetic sensor, close to the distal end, wherein: the fourth magneticsensor is configured and arranged, instead of the second magneticsensor, to be separated from the third magnetic sensor by the majorsensor separation.
 3. The probe according to claim 1, wherein magneticsensors configured and arranged to be separated by the major sensorseparation, are substantially disposed along a longitudinal axis.
 4. Theprobe according to claim 1, wherein the magnetic sensors configured andarranged to be separated by the major sensor disposition, aresubstantially disposed along the probe longitudinal axis.
 5. The probeaccording to claim 1, wherein the first and second magnetic sensors aresubstantially disposed along the transverse axis, the transverse axisbeing approximately perpendicular to the probe longitudinal axis, alongwhich sensors are configured and arranged to be separated by a majorsensor separation.
 6. The probe according to claim 1, wherein the atleast one of the first magnetic sensor, the second magnetic sensor, andthe third magnetic sensor is configured and arranged to determine one ofthe one or more dispositions of the implantable magnetic marker withrespect to the distal end of the probe.
 7. The probe according to claim1, wherein at least one magnetic sensor is configured and arranged todetermine one of the one or more dispositions of the implantablemagnetic marker in more than one degree of freedom.
 8. The probeaccording to claim 1, wherein the probe further comprises: one or morecompensation sensors for measuring a background magnetic field; wherein:the determination, in use, of one or more dispositions of theimplantable magnetic marker further considers the background magneticfield.
 9. The probe according to claim 1, wherein: the minor sensorseparation and/or the major sensor separation is predetermined byconsidering the inverse cube law determination of a magnetic fieldstrength associated with the implantable magnetic marker.
 10. A detectorunit for detecting the disposition of an implantable magnetic marker,the detector unit comprising the magnetic probe according to claim 1.11. A method for determining a disposition of an implantable magneticmarker comprising: providing a probe comprising a distal end, configuredand arranged to be disposed close to an outer surface of skin, the probeextending along a probe longitudinal axis and further comprising: afirst magnetic sensor close to the distal end and a second magneticsensor, configured and arranged to be separated by a minor sensorseparation from the first magnetic sensor, and a third magnetic sensorclose to a proximal end, configured and arranged to be separated by amajor sensor separation from the second magnetic sensor, the first andsecond magnetic sensor being substantially disposed along a transverseaxis, the transverse axis intersecting the probe longitudinal axis, thedistal end of the probe being further configured and arranged to contactan outer surface of skin and/or to be inserted through an outer surfaceof skin and/or to be inserted into a body cavity; configuring andarranging the first and second magnetic sensors to determine, in use,one or more dispositions of the implantable magnetic marker; configuringand arranging the third and second magnetic sensors to furtherdetermine, in use, the one or more dispositions of the implantablemagnetic marker; wherein at least one of the first magnetic sensor, thesecond magnetic sensor, and the third magnetic sensor are comprised inan arrangement of two or more magnetic sensors in a 1D, 2D, or 3D array,wherein the first magnetic sensor and the second magnetic sensor areco-linear along the transverse axis, and wherein the second magneticsensor and the third magnetic sensor are co-linear along an axisparallel to the probe longitudinal axis; wherein a ratio of the majorsensor separation to the minor sensor separation is in the range 1.25 to40, and wherein the probe is configured to detect the magnetic field dueto the orientation of the sensors relative to the probe longitudinalaxis, and wherein a length of the probe longitudinal axis is greaterthan a length of the probe transverse axis.
 12. The method according toclaim 11, the probe further comprises: a fourth magnetic sensor, closeto the distal end, and the method comprises: configuring and arrangingthe fourth magnetic sensor, instead of the second magnetic sensor, to beseparated from the third magnetic sensor by the major sensor separation.13. The probe according to claim 1, wherein the ratio of the majorsensor separation to the minor sensor separation is in the range of 1.6to 7.6.
 14. The probe according to claim 7, wherein at least onemagnetic sensor is configured and arranged to determine a disposition ofthe implantable magnetic marker in three degrees of freedom.
 15. Themethod according to claim 11, wherein the ratio of the major sensorseparation to the minor sensor separation is in the range of 1.6 to 7.6.